1. Field of the Invention
This invention relates to Read Only Memory (ROM) manufacturing techniques, more particularly to a method of producing high density ROM's that embody a novel technique to introduce code implantations.
2. Description of the Prior Art
Non-volatile semiconductor ROM devices are advantageous in that the information stored therein is not lost when the power supply is lost or removed. In a FET ROM, the data is written into the ROM as part of the manufacturing process. The writing process is called coding. In general a ROM device is an array of FET's arranged in columns and rows supported on a semiconductor substrate. A plurality of closely spaced line regions are provided in the substrate with a dopant of a type opposite the background conductivity dopant of the substrate. A plurality of parallel, closely spaced conductive lines are provided on the surface of the substrate that extend in a direction that is orthogonal to the line regions in the substrate. At the intersection of each surface conductive line and two adjacent line regions, an FET is formed. The adjacent line regions constitute a source and drain region, with the surface conductive line, overlying a gate oxide layer, forming the gate electrode. Each FET can be addressed separately by applying the proper voltages to the line regions, i.e. bit lines, and the surface conductive line, i.e. the word line. In the coding operation, the areas under selected word lines, between the source and drain regions, is made permanently conductive, as by introducing dopant ions into the substrate. In a FET storage cell, a binary 0 or 1 is represented by the conductive state of the FET. Arbitrarily, a 0 will be assigned to an FET that conducts between its drain and source, and 1 will be assigned to a non-conducting FET. The procedure for integrating the cells is well known.
Referring now to FIG. 1, there is illustrated a prior art technique for coding a ROM. FIG. 1 is taken on a line parallel to the bit line regions and in between the regions, so none will appear in the figure. As shown, the coding procedure arbitrarily calls for forming conductive regions 14 for the normally-off ROM or non-conductive regions for the normally-on ROM, beneath word line 10, and no region beneath word line 12 in substrate 16. A photoresist layer 18 defines opening 20 over word line 10. Since the device geometry is of such a small scale, it is not possible to align openings 20 exactly over word lines 10. The resultant conductive region 14 are not located exactly beneath the lines 10, and in some instances, due to ion migration, may meet, causing a defective reading.
What is desirable is a more precise procedure for forming the conductive for the normally-off ROM or the non-conductive regions for the normally-on ROM regions during the coding operation.